The present invention relates to a printer such as a dot matrix printer.
Wire-dot-impact printers have a carriage slidably movable on a guide rod along a platen, a print head for projecting and retracting print wires from and into its distal end, and an ink ribbon. A sheet of print paper is set on the platen and fed as the platen rotates about its own axis. Then, the print wires are selectively controlled to strike the sheet of print paper through the ink ribbon to thereby print desired information on the sheet of print paper. In order to achieve appropriate printing in such printers, it is necessary that a suitable gap be present between the sheet of print paper and the distal end (print wires) of the print head.
Recently, there has been developed a printer which incorporates an adjusting mechanism for automatically adjusting the gap between the sheet of print paper and the print head. The adjusting mechanism includes a pressing/releasing mechanism for moving the carriage toward and away from the platen through angular movement of the guide rod about an axis which is displaced eccentrically off the geometric central axis of the guide rod. Before a printing process is carried out, a ribbon mask on the distal end of the carriage is pressed under predetermined pressure against the sheet of print paper on the platen by the pressing/releasing mechanism, and thereafter the carriage is moved a suitable distance away from the platen by the pressing/releasing mechanism.
When the printer is shipped from the factory, a control device comprising a microcomputer or the like, which controls the printer, stores as a reference position the position of the carriage at the time the ribbon mask is pressed against the platen under a pressure which is the same as the above predetermined pressure. The thickness of the sheet of print paper is detected as the difference between the reference position and the position of the carriage when the ribbon mask is pressed against the sheet of print paper. The carriage is displaced from the sheet of print paper by a preset distance which corresponds to the detected thickness of the sheet of print paper. Therefore, the print head can print desired information on any of various sheets of print paper having various thicknesses, while being spaced from the sheet of print paper by a distance suitable for printing.
When the printer is continuously driven for a long period of time, a considerable amount of heat is generated from a driver of the print head, causing to increase the temperature in the interior of the printer. As the temperature increases, the parts of a printing mechanism are thermally expanded, particularly the print head (print wires) and the rubber-made platen are thermally expanded to a great extent. Due to the thermal expansions of the print head and the platen, the gap between the sheet of print paper and the print head which has been adjusted by the adjusting mechanism is shortened.
If the parts of the printing mechanism are largely deformed due to the thermal expansions, the gap between the sheet of print paper and the print head which has been set to an optimum value varies, with the result that the density of printed information on the sheet of print paper varies, and at worst the printing cannot be achieved appropriately.